Effective grounding is essential to:
- Prevent the enclosure of an electrical equipment and its connected conductors rising to a potential dangerously above that of its surroundings or ground. If the environment has a risk of an explosion, there may be a danger of sparking from very small voltage differences.
- Allow sufficient ground fault current to safely flow to the grounding system to operate the protective devices without danger. This requirement may conflict with the necessity to keep potentials at a low level and restrict the available methods of protection. The "earth loop impedance" of the completed system to be measured in order to ensure protection operation is not compromised.
- Suppress dangerous earth potential gradients.
To ensure the benefits of effective grounding, it is necessary to determine the degree of grounding provided in the system by comparing the magnitude of ground-fault current to the system three- phase fault current. The higher the ground-fault current in relation to the three-phase fault current the greater the degree or effectiveness of grounding in the system.
IEEE Std 142 defines effectively grounded as
Grounded through a sufficiently low impedance such that for all system conditions the ratio of zero-sequence reactance to positive-sequence reactance ( X0 / X1 ) is positive and not greater than 3, and the ratio of zero-sequence resistance to positive-sequence reactance ( R0 / X1 ) is positive and not greater than 1.
Effectively grounded systems will have a line-to-ground short circuit current of at least 60% of the three-phase short-circuit value. In terms of resistance and reactance, effective grounding of a system is accomplished only when R0 ≤ X1 and X0 ≤ 3X1 and such relationships exist at any point in the system. The X1 component used in the above relation is the Thevenin equivalent positive-sequence reactance of the complete system including the sub-transient reactance of all rotating machines.
Other reading materials for effective grounding
- NFPA 70 - National Electrical Code
- IEEE 142 - IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems
- NFPA 99 - Health Care Facilities Code
- ANSI/SCTE 275 2021 - Electrical Grounding and Bonding for Cable Broadband Network Critical Facilities
- IEEE 80 Guide for Safety in Alternating-Current Substation Grounding
- IEEE 1100 Powering and Grounding Sensitive Electronic Equipment